This invention relates to methods and apparatus for improving process efficiency of the steam reforming of hydrocarbons (SRH) method of hydrogen gas production. More particularly, the process improvements use a Coriolis flowmeter to control the steam to carbon ratio in SRH hydrogen gas production.
Hydrogen is an increasingly valuable commodity having many uses, such as coolant in electrical equipment, fuel for space exploration, and in chemical manufacturing of commercially important products, especially ammonia, methanol, oxo alcohols and hydroformed gasoline. Hydrogen demand is increasing due to regulatory requirements that spur the development of better performing and cleaner fuels.
The primary method of producing hydrogen in commercial quantities is steam reforming of hydrocarbons (SRH). The process may be performed on hydrocarbon gasses or low-octane petroleum fractions under process conditions that typically involve high heat and pressure. Where the reformation process is performed without a catalyst, it is generally known in the art as thermoforming. SRH is most efficient when a catalyst, such as nickel, molybdenum or platinum, facilitates the reaction. A low sulfur hydrocarbon feedstock is needed to avoid poisoning the catalyst. SRH is well known in the art and is described in a variety of publications, such as R. N. Shreve, Shreve""s Chemical Process Industries, McGraw-Hill, Inc., pp. 106-109 (1984); and D. M. Considine, Chemical and Process Technology Encyclopedia, McGraw-Hill, Inc., pp. 592-596 (1974), which are hereby incorporated by reference to the same extent as though their content is fully repeated herein.
Hydrogen gas production by the SRH method involves reacting a hydrocarbon feedstock with steam. In general, hydrocarbon feedstocks contain a variety of hydrocarbons, and the reaction chemistry proceeds according to ideal stoichiometric equations for each type of hydrocarbon. A variety of different reactions occur, depending upon the feedstock. The most important reactions can be generally categorized as:
A. Dehydrogenation of cyclohexanes to yield aromatic hydrocarbons;
B. Dehydrogenation of certain paraffins to yield aromatics;
C. Isomerization including the conversion of straight-chain to branched chain carbon structures, such as octane to isooctane;
D. Reformation of methane in natural gas to produce carbon dioxide and hydrogen; and
E. Reformation of naptha to yield synthetic natural gas.
A preferred manner of generating large quantities of hydrogen is to use a natural gas feedstock that contains a large portion of methane. The reaction proceeds as shown in Equation 1:
CH4+H2O= greater than CO+3H2,xe2x80x83xe2x80x83(1) 
where H2O is preferably present as steam.
This class of reaction similarly operates on other gas fractions in the feedstock for complete decomposition, in an ideal sense, of the hydrocarbon into carbon dioxide and hydrogen. For example, a further reaction including propane (C3H8) in the hydrocarbon feedstock proceeds according to Equation (2):
C3H8+3H2O= greater than 3CO+7H2,xe2x80x83xe2x80x83(2) 
where H2O is preferably present as steam.
More generally, this overall class of reaction proceeds as shown in Equation (3):
CnHm+nH2O= greater than nCO+(m/2+n)H2xe2x80x83xe2x80x83(3) 
The foregoing equations predict the reaction of lighter hydrocarbons, especially, methane, butane and propane, as well as some liquids, such as naptha. Heavier ends tend to react differently and, while some proceed according to the above Equation (3), other such reactions as isomerization occur, also with resultant hydrogen production.
The equations demonstrate a concept that different amounts of steam are required to complete the reaction, depending upon the feedstock composition. For example, one mole of methane requires one mole of steam in Equation (1), whereas one mole of propane consumes three moles of steam in Equation (2). In commercial manufacturing facilities that operate upon feedstocks of various compositions, these differences impose a potentially significant materials balance problem. The process may be further complicated by using oxygen as a reagent, which results in lower steam consumption.
A wide range of feedstocks can be utilized as the hydrocarbon feedstock in a SRH hydrogen production unit. Hydrocarbons such as natural gas, methane, propane, butane and naphtha can be used as the hydrocarbon feedstock, either alone or in combination. Economics and the availability of particular hydrocarbon feedstocks may dictate the use of different hydrocarbon feedstocks from one period to another.
A particular problem arises in petroleum refineries because the SRH feedstock composition constantly changes. The hydrocarbon feedstock for a hydrogen production unit can come from several sources within the refinery, and these sources contribute different hydrocarbons. One particular example of a combined-source hydrocarbon includes the refinery fuel gas system. Numerous process results contribute to the fuel gas system by adding different hydrocarbons, which may be directed in a combined stream to the hydrogen gas production unit. If one of the contributing refinery fuel gas system processes is shut down or changes in terms of output volume, the composition of the fuel gas system output changes. The changes in feedstock composition require corresponding changes in the SRH process conditions, such as heat, pressure and flow rate, in order to optimize process efficiencies and minimize environmental pollution.
It is presently a problem to accurately measure the fractional composition of a hydrocarbon feedstock in a hydrogen production unit. It is a further problem to control the ratio of steam to carbon in a hydrogen production unit based upon the composition of the feedstock. These measurement and control problems reduce SRH process efficiencies while increasing associated environmental pollution problems.
A traditional approach to measuring the hydrocarbon feedstock in a hydrogen gas production unit involves measuring the hydrocarbon feedstock with a volumetric flowmeter. While somewhat useful in addressing the reaction balance problem arising from combined feedstocks, however, volumetric meters are incapable of providing a full measurement solution. When the composition of the hydrocarbon feedstock changes or a substitute hydrocarbon feedstock is used, the amount of carbon contributed to the hydrogen gas production unit can change dramatically within a given unit of volume, as can the required amount of steam to complete the reaction. For example, under identical conditions of temperature and pressure, the amount of steam required to react with a volume of propane is approximately three times greater than the amount of steam required to react with the same volume of methane. The problem is exacerbated by real gas behavior where the lighter gasses tend to have higher compressibility factors.
Another approach to measuring the hydrocarbon feedstock is to determine the composition of the hydrocarbon feedstock with a gas chromatograph. However a gas chromatograph cannot provide real-time composition data for a hydrocarbon feedstock.
It is known to use Coriolis mass flowmeters to measure mass flow and other information with respect to materials flowing through a pipeline as disclosed in U.S. Pat. No. 4,491,025 issued to J. E. Smith, et al., of Jan. 1, 1985 and Re. 31,450 to J. E. Smith of Feb. 11, 1982. These flowmeters typically comprise a flowmeter electronics portion and a flowmeter sensor portion. Flowmeter sensors have one or more flowtubes of a straight or curved configuration. Each flowtube configuration has a set of natural vibration modes, which may be of a simple bending, torsional, radial or coupled type. Each flowtube is driven to oscillate at resonance in one of these natural modes. The natural vibration modes of the vibrating, material filled systems are defined in part by the combined mass of the flowtubes and the material within the flowtubes. Material flows into the flowmeter sensor from a connected pipeline on the inlet side of the flowmeter sensor. The material is then directed through the flowtubes and exits the flowmeter sensor to a pipeline connected on the outlet side of the flowmeter sensor.
When there is no material flowing through a Coriolis flowmeter sensor, all points along the flowtubes oscillate with a substantially identical phase. As material flows through the flowtubes, Coriolis accelerations cause points along the flowtubes to have a different phase. The phase on the inlet side of the flowmeter sensor lags the driver, while the phase on the outlet side of the flowmeter sensor leads the driver.
Coriolis flowmeter sensors typically include two pick-offs for producing sinusoidal signals representative of the motion of the flowtubes at different points along the flowtubes. A phase difference of the sinusoidal signals received from the pick-offs is calculated by the flowmeter electronics. The phase difference between the pick-off signals is proportional to the mass flowrate of the material flowing through the flowmeter sensor.
Coriolis flow measurement systems have not been adapted for use in SRH processes, in part, because they fundamentally measure mass, as opposed to the conventional volumetric systems. Additionally, it has not been understood how mass flow meters could be adapted to measure or estimate the mass attributable to fractional components of the combined feedstocks.
The above and other problems are solved and an advance in the art is made by adapting Coriolis flowmeters for use measuring the hydrocarbon feedstock of a SRH hydrogen gas production unit. The use of Coriolis mass flowmeters, as described hereinbelow, results in a more accurate, versatile and real-time hydrocarbon feedstock measurement than is permitted by conventional systems. Further, the use of a Coriolis effect mass flowmeter allows for increased control of the carbon to steam ratio in hydrogen gas production.
In general terms, the metering system and method provided in accordance with the present instrumentalities uses a mass flowmeter, such as a Coriolis mass flowmeter, to measure the mass or mass flow rate of the hydrocarbon feedstock delivered to a hydrogen gas production unit. The mass flowrate of the hydrocarbon feedstock is then used to control the steam to carbon ratio in SRH hydrogen gas production.
One such embodiment of a mass flowmeter system comprises a hydrocarbon feedstock supply for supplying a hydrocarbon feedstock to the hydrogen gas production system. A steam supply is used to supply steam to the hydrogen production system. A mass flowmeter is operably connected to the hydrocarbon feedstock supply for measuring a hydrocarbon mass flow rate therein and for producing a hydrocarbon flow rate signal representing the hydrocarbon mass flow rate. A second flowmeter is operably connected to the steam supply for measuring a steam flow rate and for producing a steam flow rate signal representing the steam flow rate. A controller is operable for receiving the hydrocarbon flow rate signal and the steam flow rate signal. The controller has program instructions for controlling a ratio of the hydrocarbon feedstock and the steam delivered to the hydrogen production system.
Preferred embodiments include one or both of the mass flowmeter used in the hydrocarbon supply line and the second flowmeter used in the steam supply line comprising Coriolis mass flowmeters. These instrumentalities may be used to particular advantage where the hydrocarbon feedstock comprises a mixture of hydrocarbon gasses, or hydrocarbons materials having a different composition over a period of time. Overall accuracy of the system is improved by the use of a correlation to determine the carbon content in the hydrocarbon feedstock. Accuracy is further enhanced by measuring a physical parameter of the hydrocarbon feedstock, such as density or gas gravity, to facilitate the correlation.
A method of operating the previously described system includes, for example, measuring a mass flow rate of a hydrocarbon feedstock delivered to the hydrogen production system and provide a hydrocarbon mass flow rate measurement, measuring a second flow rate of steam delivered to the hydrogen production system to provide a steam flow rate measurement, controlling the amount of the hydrocarbon feedstock and the steam delivered to the hydrogen producing system based upon the hydrocarbon mass flow rate measurement and the steam flow rate measurement. The measurements may be made contemporaneously with control operations to adjust the ratio of carbon and steam. The measuring steps may even be repeated in real time while the controller is making adjustments to the respective flow rates.
The metering system and method provided in accordance with the present instrumentalities may utilize a CPU in the form of meter electronics, a controller or any other computational device, which operates on a signal representing the hydrocarbon mass flow measurement and controls a valve or other device capable of modifying the hydrocarbon flow according to the desired carbon to steam ratio. The computational device may also use these signals to control a valve or other device capable of modifying the steam flow according to the desired carbon to steam ratio. The Coriolis mass flowmeter metering system is particularly advantageous when a hydrogen gas production unit readily switches between different types of hydrocarbon feedstocks or receives hydrocarbon feedstocks of modified hydrocarbon composition.
In accordance with the discussion above, the disclosed instrumentalities have the following object, aspects, and advantages.
It is an aspect of the invention to adapt a mass flowmeter for use in SRH processes.
It is a further aspect of the invention to obtain the benefit of accuracy inherent to Coriolis flowmeter systems and apply the same to SRH processes.